This invention relates to a novel class of ion exchange and chelation polymers which contain dendritic branches having ion exchanging or chelating moieties uniformly distributed on the periphery of such branches. The invention also relates to processes for using such polymers as ion exchange resins and chelate resins.
Ion exchange resins are widely used to remove ions from aqueous media, and for a wide variety of other purposes. For example, such resins are used in water treatment including softening brackish waters, dealkalization, demineralization, condensate polishing, and nitrate removal; waste treatment such as treatment of radioactive waste streams, chromate recovery and rinse water recycling. Such resins are also useful in chemical processing such as catalysis purification, metal extraction, ion-exclusion purification, ion retardation, and desiccation; sugar separation and purification such as glucose/fructose separation and sucrose decolorization and deashing; and chemical analysis. Cation exchange resins ar those having a negatively charged matrix and exchangeable positive ions, e.g., strong acid resins such as sulfonated polymers of styrene crosslinked with varying degrees of divinylbenzene and weak acid resins such as polymers of acrylic acid or methacrylic acid that are crosslinked with divinylbenzene. Anion exchange resins have a positively charged matrix and exchangeable negative ions, e.g., strong base resins such as quaternary ammonium salt forms of polyvinyl benzyl chloride which has been crosslinked with divinylbenzene and weak base resins such as epichlorohydrin amine condensates that bear primary, secondary or tertiary amine moieties. The most effective ion exchange resins have good capacities, i.e., the ability to exchange large numbers of ions before needing to be regenerated. Such resins also should have good crush strength, i.e., the ability to resist breakage resulting from handling or osmotic shocks which often occurs during regeneration.
While the standard ion exchange resins are effective for essentially total removal of ions from solutions, they generally do not function well in the selective removal of heavy metal ions from solutions containing significant quantities of alkali and alkaline earth metal ions. Accordingly, chelate resins have been developed to selectively remove the heavy metal ions from such salt solutions. Chelate resins are normally solid chelating agents which have the ability to extract metal ions from a liquid without substantial structural alteration of the solid resin. The most effective chelate resins possess the capacity to chelate with a large number of metal ions before the need for regeneration, i.e., preparing the resin for reuse by displacing the metal ions removed by the resin. They also advantageously exhibit a high porosity and are resistant to physical deterioration such as excessive swelling or shattering. Moreover, to obtain maximum benefit of the resin's properties, a spheroidal particle size is often desirable.
Heretofore, chelate resins have been conventionally prepared by the addition of chelate active functional groups to an insoluble resin matrix such as a crosslinked vinyl aromatic polymer, e.g., a crosslinked polystyrene. A method for adding chelate active groups to such resins by the sequential steps of halomethylation amination and carboxylation is disclosed in Ion Exchange, by F. Helfferich, published in 1962 by McGraw-Hill Book Company, New York. Alternatively, the chelating groups are added to such resins by the reaction of halomethylated resin with an appropriate reactant such as a nitrile-containing amine and the hydrolysis of the reacted product. See, for example, U.S. Pat. No. 3,043,809 to Mattano. Unfortunately, such methods require numerous process steps, each of which require relatively exacting control to prevent unwanted side reactions. Moreover, the resins prepared by such methods possess relatively low chelate stability constants with many multivalent cations.
In view of the extensive use of ion exchange resins and chelate resins on a commercial scale, it is highly desirable to provide new resins that are more efficiently produced and/or used.